Prepare to be confused--(note: i could be completely wrong, but i'm pretty sure i'm right)-----
Ok, first, in Japan, Green (well, Green to us Europeans and Americans, Blue to the Japanese) was released. It was not released anywhere else. Green (jap: Blue) had more complex caves and routes, terrible graphics and all 150 Pokemon available in the one game.
Next came Red and Blue (Blue being called Green in Japan). These were released worldwide, with improved graphics, less complex caves and routes, and required people to buy both versions/trade if they wanted to get all 150 Pokemon.
Next came Yellow version, which was also released worldwide. It was as all 3rd sister games are- slightly different events, different Pokemon Locations etc.
In a slightly complicated conclusion, Yellow is the ''master game''- Green (jap: Blue) was kinda forgotten, because of the flaws, so the ''true'' trilogy for Europe and America are Red, Blue and Yellow (RBY), and the ''true'' trilogy in Japan is Red, Green and Yellow (RGY), the only difference between the two trilogies being the names (Green->Blue).
Hopefully i'm clear here.
(Also, hopefully i'm right...).